1. Field of the Invention
The present invention relates to an electric power steering apparatus for assisting a steering torque generated by a vehicle driver's steering operation.
2. Description of Related Art
There is known an electric power steering apparatus which includes a plurality of power converters provided corresponding to a plurality of winding sets of a motor, and is configured to continue to supply power to the motor even when one of the power converters or the corresponding winding set fails by using the remaining normal power converter(s) and winding set(s). For example, the electric power steering apparatus described in Japanese Patent Application Laid-open No. 2011-131860 is configured to, upon detecting a failure in one of power converters or the corresponding winding set, seta current limit value of the remaining normal power converter(s) to a value equal to a current limit value having been set for the power converter(s) before detecting the failure. For example, in a case where the electric power steering apparatus includes two power converters, and one of the two power converters fails, the electric power steering apparatus continues to operate while setting the current limit value of the remaining one normal power converter to a half of the sum of the current limit values having been set for the two power converters before the occurrence of the failure.
In this case, since the sum of the current limit values after the occurrence of the failure becomes half of that before the occurrence of the failure, the vehicle driver can notice a change in the force necessary to operate the steering wheel, and accordingly notice occurrence of some failure. In addition, the electric load of the normal power converter can be prevented from becoming excessive.
Incidentally, an assist torque required to an electric power steering apparatus varies depending on the speed of a vehicle. That is, when the vehicle is running at a low speed, it is preferable to increase the assist torque so that the steering torque to be generated by the vehicle driver becomes smaller and accordingly the vehicle driver can maneuver the vehicle easily. On the other hand, when the vehicle is running at a high speed, it is preferable to reduce the assist torque so that the steering torque to be generated by the vehicle driver becomes larger in view of ensuring the straight-running stability of the vehicle. Hence, generally, conventional electric power steering apparatuses are configured to perform a vehicle speed responsive control in a normal state, in which a relationship between the assist current (the motor current) and the steering torque is determined in accordance with the vehicle speed. However, the electric power steering apparatus described in the above patent document cannot operate taking into account the vehicle speed when a failure occurs in any one of the power converters or the winding sets of the motor. Accordingly, the electric power steering apparatus described in the above patent document cannot operate optimally when a failure occurs in any one of the power converters or the windings sets of the motor.